Shane won’t be the best ever till he starts beating the top players in 1P. Efren has beaten the top players in almost every game.
One pocket is kind of a regional one-off game. Yes if people want to gamble big in the US they need to learn it.
But saying that accomplishment in one-pocket has something to do with being the best overall player is not reasonable because just not enough people care about it. It is a specialized subskill and there are many many countries and whole continents that don't care about it.
I would have the same objection if a Canadian said accomplishment at Hockey must be a fundamental part of what determines who are North America's best athletes.